a) Field of the Invention
The invention is directed to a video projection system for projecting more than one picture in which a light source for generating at least one light bundle, means for intensity modulation and means for spatial light bundle modulation, wherein these means are controlled by a video signal, and at least one projection surface for displaying a determined quantity of individual pictures are arranged in the direction of light of a beam path.
b) Description of the Related Art
There are known projectors with light sources which can derive from two fundamental principles:
1. Projectors based on the conventional imaging principle, e.g., LCD projectors and DLP projectors. These projectors will be referred to hereinafter as "picture-imaging projectors"; PA0 2. Projectors working with light bundles which are deflected in two dimensions. These projectors are also referred to, for example, as "laser display" or "laser projectors".
The above-mentioned projectors can be designed exclusively for showing monochrome pictures or also for showing black-and-white pictures. In these cases, it is sufficient to convert the electronic picture information into an intensity modulation or brightness modulation of the light. The picture impression depends first on the adjustment of picture brightness with reference to the ambient light conditions. The maximum picture brightness is limited by the light output which is modulated by 100%. The picture impression is further determined by the level of contrast. In adjusting the contrast, the modulation amplitude is fixed; that is, the gray values of the video signal are correlated with correspondingly modulated light outputs.
If a plurality of projectors of the same type which are arranged adjacent to one another show identical pictures, it is only necessary to adjust the parameters of brightness, contrast and spectral intensity distribution of the projectors relative to one another. With four projectors for four pictures, this means that there could be 12 possible adjustments.
Further, it must be ensured that the means for picture modulation and associated controlling electronics in the projectors operate with entirely identical characteristics. Only in this way will it be achieved in practice that a plurality of projectors receiving the same video information also show the same pictures.
The degrees of freedom for adjusting the projectors are increased almost three-fold when generating color images. In the case of a color picture projector, this can be explained by the fact that three different colored monochrome systems are put together in practice: a system for the primary color red, a system for the primary color green, and a system for the primary color blue. Accordingly, there are 9 possibilities for adjusting the color effect, contrast and brightness of a picture. Therefore, with four projectors, there are already 36 degrees of freedom for adjustment! The following picture properties relevant for the human faculty of sight must be extensively identical in a plurality of projectors: contrast (light-dark ratio), gradation (channel gradation), convergence (color overlap), picture sharpness, picture composition (pixel structure), brightness (luminous density), color intensity (saturation) and color shade (hue). In particular, color differences (differences in color stimulus values) between projectors which project pictures directly adjacent to one another are determined in an extremely sensitive manner by a human observer. The objective of matching the projection characteristics of a plurality of projectors to the greatest possible extent can be achieved in practice only at considerable expense with respect to technology, cost, personnel and time.
In order to project large pictures, a plurality of individual pictures are usually combined to form a total picture. For this purpose, each individual picture is generated by a complete projector.
Arrangements comprising a plurality of LCD projectors, for example, are known. According to EP 0 731 603 A2, each of these projectors contains a white light source from which the three primary colors, red, green and blue, are filtered out. Constructions containing three light sources, each of which provides a primary color, are also known. These light sources are temperature radiators. Solutions making use of temperature radiators have substantial deficiencies. The observer of the projected picture expects to be presented with a picture of uniform brightness and color depiction. However, this cannot be realized when each projector generating an individual picture has its own light source. It cannot be expected that the brightness of the independent light sources of each projector and their color-reproducing characteristics will really be identical and that they will actually remain identical permanently.
A primary problem consists in that the projection lamps age differently and unforeseeable changes in the spectral intensity distribution occur through this process. Tests have shown that the color temperature of individual projection lamps, even of the same type and age, show substantial differences in radiating characteristics. As a result, the individual projectors will reproduce different colors for the individual picture in question. In the course of aging, the color temperatures of the individual projection lamps change very differently.
Another source for differences in the spectral intensity distribution are differences and tolerances in the supply of power to the individual light sources or cathode ray tubes of the projectors. All of these negative influences lead to a considerable waste of time and material on the part of the operator of an installation of this kind to produce and maintain a balance of brightness and color effect in the individual projectors. In practice, the picture composed of the individual images must satisfy at least modest requirements with respect to quality. The process of matching the individual projectors is very time-consuming and complicated. For example, if brightness is increased by changing the supply voltage or the supply current to a projection lamp, the color temperature also changes. This influence can only be compensated by a new color matching of the projector. When a projection lamp fails and is replaced with a new one, this process becomes still more uneconomical as the new projection lamp will differ substantially with respect to light output and color temperature from those that have already been in use for a long period of time. Accordingly, it is often necessary to replace all of the projection lamps in the system at the same time when one projection lamp fails. Again, this increases costs.
Further, It is known from EP 0 589 179 that the projection lamps (temperature radiators) in picture-imaging projectors can be replaced with laser light sources. Laser light sources have the advantage over temperature radiators that they can emit light in defined wavelengths, i.e., different laser light sources of one type emit light of exactly the same wavelength. This is a great advantage, for one, in that the spectral intensity distribution is identical in laser light sources of the same construction.
However, difficulties arise even in this case with a monochrome projection system, and increased technical expenditure is required to operate a plurality of laser light sources for the individual pictures in such a way that the total picture gives a uniform impression of brightness.
A picture-imaging laser projection system generating color pictures also needs three laser light sources in the primary colors red, green and blue for each projector. One example of this is a DLP system according to EP 0 589 179 A1 in which three laser light sources generate laser light bundles of different wavelengths. The light bundles are spatially combined and expanded to the size of a DMD array by means of optics. A projection objective images the DMD array on the projection surface.
In a multiple arrangement of these projectors, the ratios of the intensities of the three laser light sources of a projector relative to one another and the maximum degree of intensities (picture brightness) and modulation amplitude (contrast) of each of the projectors must be exactly matched to one another. Special problems are caused in this respect by expanding every laser light bundle to the size of the DMD array in order to achieve a homogeneous, uniformly strong illumination of the entire surface of the internal object planes of the individual projectors.
Although it might be assumed in this case that the defined wavelengths of the laser light sources would simplify adjustment, a considerable effort is also required in this case to balance the red-green-blue laser light sources of a plurality of projectors in such a way that all pictures have approximately the same reproduction characteristics for a color picture.
Further, projectors are known (e.g., U.S. Pat. No. 5,424,771) which work with a writing light bundle. A main advantage of these laser projectors over picture-imaging projectors consists in that a virtually parallel laser light bundle forms the picture in the line dimension, so that optical imaging of a picture subject from an object plane into an image plane is not carried out. However, the problems of adjusting identical reproduction characteristics in a plurality of projectors are only slightly improved over those described above. The main advantage operative in this respect is that projectors working with a writing light bundle do not have object planes internal to the device, so that the problem of a uniform illumination of an object plane does not arise. Projectors of this type have the further advantage of a virtually unlimited depth of focus, so that pictures are always sharp with these projectors.
However, tests have also shown that a plurality of projectors, although controlled by the same video signal, show visible differences in picture reproduction which can also only be reduced by relatively elaborate measures by the nine degrees of freedom in adjustment per projector.
The difficulty in achieving matching projection characteristics in a plurality of projectors is due particularly to the differences between the beam parameters of the laser light bundle of a plurality of identical laser light sources. These parameters are essentially the light bundle diameter, divergence, polarization state and mode structure. In this case also, these parameters can be brought into agreement for different laser light sources to an extent sufficient for maintaining an identical appearance of a plurality of pictures only by additional effort. Another problem is that the working stability of each individual laser light source must be ensured over a longer period of time.
In simulators for trucks, helicopters, aircraft and ships, pictures are projected on cylindrical or spherical surfaces in order to simulate the horizontal image angle as it would actually be seen in a real-life situation. Because of the large viewing angles required for achieving a realistic picture impression, a picture, in this case, is composed of a plurality of individual pictures coming from different projection devices.
U.S. Pat. No. 4,297,723 shows an arrangement for showing pictures in the above-mentioned manner with a projector. Three laser light bundles in red, green and blue are intensity-modulated separately for a left-hand individual picture, a right-hand individual picture and a center individual picture. The intensity-modulated light in red, green and blue of each individual picture is combined spatially by means of chromatic mirrors. Three adjacent parallel light bundles are formed, each of which contains the picture information for an individual picture. These light bundles are deflected together in two dimensions with a biaxial scanning system. The three light bundles are then deflected onto a curved projection surface by specially constructed projection optics in such a way that the three individual pictures result in a total picture. The projection of the center picture is carried out in the direction given by the scanning system. The right-hand individual picture and the left-hand individual picture are spatially separated from the center picture by deflecting mirrors. These mirrors must be adjusted very exactly to the axes of the principal projection beams in order to obtain a total picture of reasonably seamless composition. The projection angles cannot be substantially greater than 50.degree. because of the deflection mirrors needed for dividing up the picture.
For technical reasons, it is not possible in most cases to realize a picture projection, e.g., for a horizontal viewing angle of 200.degree., with only one projection device.
Two (e.g., EP 0 210 088 B1), three, four, or sometimes even more projection channels (e.g., EP 0 522 204 A1 or U.S. Pat. No. 5,424,771) are required, each of which projects an individual picture, wherein the individual pictures are joined to form a total picture. For this purpose, the prior art uses as many projectors and as many light sources as required by projection channels. The arrangement of the projectors is carried out in such a way that the pictures can be placed together as required by the picture projection.
According to EP 0 522 204 A1, six projectors are located in the center of a projection dome. In this case, according to FIG. 31, six or even more independently operating red-green-blue laser light sources are used, each of which contains, in turn, independently operating laser light sources in the primary colors. A plurality of projectors are used, each of which has an independent power supply and control for the lasers.
As will be gathered from the preceding description, it is absolutely necessary, based on the present level of development, to use multiple arrangements of projectors for very large pictures, especially for projection over the entire surface area of domes. However, arrangements of this type cause disproportionate increases in cost which are brought about in particular by the expenditure on installation and maintenance for projection systems with independent light sources.